Large spectral bandwidth, U.V. solar blind detector

ABSTRACT

An early warning system for detecting the UV from a missile plume has a wide field of view, large spectral bandwidth, solar blind detector. A coated detector passes only a spectral region that embraces UV signals of interest and a wavelength shifter includes a material that shifts the impinging UV energy into a spectrum that embraces the frequencies emitted by fluorescent photons. A photomultiplier tube responsive to the fluorescent emissions provides a responsive read-out indicative of an incoming missile.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

Strike and interceptor aircraft operating in a hostile environment needa wide variety of increasingly sophisticated devices to assure theirsurvival. Air or ground launched radar or infrared guided rocketpropelled missiles can take a fearsome toll if their presence is notdetected soon enough. When an early enough detection of these incomingmissiles positively can be made, evasive action, flares, electroniccounter measures, etc. can and greatly do reduce their effectiveness.

Early warning systems that detect UV from the missile plume requirelarge area, wide field of view detectors. In addition to the need forbeing highly reliable and compact in size it is highly desirable thatthe detectors be sensitive in the region of the missile plumes+ UVradiation, from 230 to 280 nanometers, to reduce false alarms.Contemporary filters that use absorption type materials are expensivesince they require large single crystals of nickel sulphate or the like.In addition, the solar-blind photomultiplier tubes that have been usedwith the large crystals also are expensive. As a consequence, technologyand funding constraints may limit aircraft from having an appropriatedetector in some high technology combat zones.

Thus, a continuing need exists in the state of the art for a large area,wide field of view detector of the UV radiation in a missile plume toassure the timely warning of an incoming missile.

SUMMARY OF THE INVENTION

The present invention is directed to providing a means for detecting theUV radiation of an incoming missile plume. A wide field of view detectorhas a coating that blocks some impinging radiation yet passes the UVradiation of interest that is radiated from an incoming missile plume. Amaterial within the detector is stimulated by the incoming UV radiationto shift to fluorescent emissions which are outside the passband of theUV radiation. A second coating on an opposite wall of the detectorpasses the fluorescent emissions to a photomultiplier tube sensitive tothe fluorescent emissions.

A prime object of the invention is to provide an improved detector of UVradiation emanating from a missile plume.

Another object is to provide a UV radiation detector having a pair ofcoatings on opposite side of a container filled with a materialsensitive to impinging UV radiations to shift to responsive fluorescentemissions.

Still another object of the invention is to provide for an improveddetector of the UV radiation emanating from a missile plume that reliesupon less costly photomultiplier tubes sensitive to radiation in thefluorescent spectrum.

Yet a further object is to provide for an improvement in a UV detectorthat has a wide area, wide field of view detection capability.

Still yet another further object is to provide for an apparatus for UVdetection that is compact and of reduced cost making it attractive forwide spread application.

These and other objects of the invention will become more readilyapparent from the ensuing specification and appended claims when takenin conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the principle constituents of theinvention.

FIG. 2 depicts the passbands of the detector coatings S₁ and S₂.

FIG. 3 shows an energy level diagram with electronic transmissions ofpotassium.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and in particular FIG. 1, a large area,wide field of view detector apparatus 10 has been designed to detect theUV radiation that is emitted from the missile plume of an incomingmissile. A hollow container 15 is disposed adjacent to a photomultipliertube 30. The pillbox-like container is fabricated from a section 16 of aquartz tube that has a pair of quartz plate caps 17 and 18 fused inplace to form a cylinder closing a chamber 19. The container has a duct,not shown, to allow introduction of a gas as will be elaborated on belowand suitable valves and piping must also be provided to allow theintroduction of the gas into chamber 19. An upper surface of cap 17 isprovided with a coating S₁ which passes only a spectral region Δλ₁ thatlies between λ₁ and λ₂, see FIG. 2.

The transmission characteristic of the material S₁ is depicted as beingsquare in shape. This is for purposes of demonstration only, it beingrealized that a certain amount of curvature is inherent. The coating canbe a laminate of a first layer which has a low pass characteristic and asecond layer which has a high pass characteristic. Both of thesecharacteristics are known to be curved, however; for purposes ofclarification, the passband is depicted as being square with lower andupper limits of 230 nm and 280 nm. In other words only solar blindenergy can enter into container 15.

Coatings having a bandwidth capability as called for above are wellwithin the purview of the current state of the art. Numerouslaboratories provide such coatings by conventional vapor depositiontechniques once the desired passbands are known. A typical laboratoryhaving such a capability is the Optical Coating Laboratory Incorporatedin Santa Rosa, Calif. This laboratory routinely provides such coatingsupon request. Other laboratories are readily available nation wide toprovide similar services.

An outer surface on cap 18 is provided with a coating S₂. This coatingtransmits only the wavelength region from λ₃ to λ₄, a spectral regionΔλ₂. This region encompasses all emitted fluorescent photons but doesnot overlap the passband Δλ₁. Since transmission bands Δλ₁ and Δλ₂ donot overlap, container 15 is completely opaque to all wavelengthsincident on it.

In this case, the bandwidth Δλ₂ may span a range of between 740 nm and790 nm. The coating S₂ like coating S1 is fabricated by a suitablyequipped lab in accordance with well established techniques to providethis passband.

Operation to provide a responsive signal at the output of photomultiplertube 30 requires that there be a wavelength shifting medium withinchamber 19. In other words, photons of UV energy passing through coatingS1 are shifted to longer wavelengths by the proper medium contained inchamber 19. This wavelength shifting or converting material may be a gasliquid or a solid and should have the following characteristics. Firstit absorbs all the photons that pass through surface S1. Second itfluoresces with large values of quantum efficiency with emission of afew wavelengths. Third the spectral region Δλ₁ should be removed fromthe spectral region Δλ₂ for ease of discrimination. For this reasoncoating S2 applied to the surface of cap 18 has a passband Δλ₂ thatencompasses all the emitted fluorescent photons of the materialcontained within chamber 19. Photomultiplier tube 30 behind coating S2is selected to be sensitive only to the spectral region Δλ₂.

Potassium vapor 35 may be selected as the wavelength shifting medium andis diffused in chamber 19. The vapor can be diffused in chamber 19 bymethods well known in the art and elaboration at this point would onlybelabor the obvious. All the photons in region Δλ₁ have energies between4.42 and 5.39EV. As these values are larger than the 4.34EV ionizationenergy of potassium all the UV photons will be absorbed. The fluorescentemission from the potassium vapor consists mainly of the doublets 769.9and 766.5 nanometer. Appropriately selecting the passband of surface S2to embrace a passband Δλ₂ extending from 740 nm to 790 nm will transmitthe fluorescent emission of the potassium vapor medium for the PMT. Atypical photomultiplier tube that can be used is a HamamatsuRed-Enhanced Multiply-Alkali photocathode type R712. FIG. 3, in itsdepiction of the energy level diagram of potassium, shows that the majorpermissible electronic transitions are indicated by vertical lines withthe emission wavelengths printed in the center of the line. This type ofelectronic jump occurs over very narrow energy spread, as the energylevels of the ground state and the excited state are very narrow(assuming the pressure is not too high). Thus, the emission andabsorption lines for these states have a very narrow spectral width.

On the other hand, the ionized state of the K atom, indicated by thehorizontal line at 4.34 is a group of contiguous, continuum states.Thus, the spectral width of this transition will be very broad. Oneboundary of this width is the minimum energy to ionize the atom, 4.34e.v., (285.6 nm photons). The other end of this width will extend intothe continuum, say to 5 e.v. a photon energy of 248 nm.

The ionized atom decays to the ground state by photon emission at 769.9or 764.5 nm. These two transitions are the strongest in the potassiumspectrum.

In other words, the K vapor allows a relatively broadband absorption ofUV energy over a broad spectral range (about 240 to 280 nm). From thishigh ionized state, the atoms decay with a subsequent emission at the769.9 and 766.5 nm, as fluorescent emissions.

Inclusion of coating S1 by itself eliminates some spurious impingingenergy photons to contribute to more efficient operation. The shiftingof the UV radiation by the potassium vapor into the fluorescent spectrumand passing of this spectrum through layer S2 further blocks spurioussignals from reaching photomultiplier tube 30 to further avoid creationof erroneous signals. Only the fluorescent radiation which is shiftedfrom the UV spectrum has any effect on the output signal of thephotomultiplier tube. Cost effectiveness is assured by substitution ofthe fluorescent sensitive photomultiplier tube as opposed to a UVspectrum photomultiplier tube. This is because fluorescent sensitivephotomultiplier tubes are less complicated to build and, as aconsequence, their per unit costs are reduced.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. An apparatus for detecting the UV radiation inmissile plume comprising: a photomultiplier tube responsive tofluorescent emissions to provide a representative output signal; adetector having a first surface covered with a first coating havingproperties to pass the UV radiation and a second coating disposedadjacent the photomultiplier tube having properties to pass thefluorescent emissions, the passbands of the coatings being separate anddistinct one from the other, the detector further having a materialtherein having the properties for shifting the wavelength of the UVradiation to that of the fluorescent emissions.
 2. An apparatusaccording to claim 1 in which the material of the detector is potassiumvapor responsive to absorb UV radiation and to transition to a longerwavelength and the first surface presents a wide area, wide field ofview UV responsive surface.
 3. A method of detecting the UV missileplume within the solar blind UV spectrum comprising: blocking outsignals other than a passband including the UV missile plume signalswith a first coating; shifting the frequency of the plume signals to afluorescent bandwidth outside of the solar blind UV spectrum; blockingout signals other than the fluorescent bandwidth outside of the solarblind UV spectrum with a second coating; and amplifying the fluorescentbandwidth.
 4. A method according to claim 3 in which the step ofshifting includes the transition of UV wavelengths to longer wavelengthsfluorescent emission.
 5. A method according to claim 4 in which the stepof transition includes providing a potassium vapor to receive theinformation signal passband to assure the transition to the longerwavelengths of fluorescent emission.